Pulmonary arterial hypertension (WHO Group I) is a rare disorder associated with poor survival. Endothelial dysfunction resulting from 1) genetic susceptibility, and 2) a triggering stimulus that initiates pulmonary vascular injury, the two-hit hypothesis, appears to play a central role both in the pathogenesis and progression of PAH. Inflammation appears to drive this dysfunctional endothelial phenotype, propagating cycles of injury and repair in genetically susceptible patients with idiopathic IPAH and patients with DaPAH. However, despite mounting evidence of vascular inflammation in patients with PAH, detailed phenotypic studies are lacking on the temporal evolution of this process and its contribution to right ventricular (RV) and pulmonary vascular remodeling. The protocol was initially approved by the NHLBI IRB in October 2012. Subsequently IRB approval was sought and obtained at collaborating sites and various amendments were brought into alignment at all institutions by summer 2013. In 2013 -15 we established at the NIH a clinical research PAH program consisting of a multidisciplinary medical staff with expertise in cardiology, pulmonary, and critical care as well as a regulatory and research nursing support staff with extensive experience in assuring compliance with clinical protocols while maintaining the highest standards of clinical care. Referrals are currently being received from multiple sites. To date 70 subjects have been screened for this study and 24 have been enrolled. Patients are thoroughly characterized by echocardiogram, 6-minute walk, cardiopulmonary stress testing, pulmonary function testing, cardiac CT, advanced cardiac MRI techniques, gene expression profiling, and serum biomarkers. We hypothesize that a detailed characterization of these patients in conjunction with a more rigorous profile of the temporal evolution of vascular inflammation in PAH and its impact on RV and pulmonary vascular function will add prognostic value to traditional measures of disease severity and suggest novel therapeutic targets for future research.